The present invention relates to a method of treating the surface of a valve lifter, and particularly, to a method of coating the outer circumferential surface of the valve lifter moulded from aluminium or its alloy, with wear resistant material.
In a valve lifter or tappet for use in a direct acting valve gear of an internal combustion engine, its body was conventionally moulded from iron such as steel or cast iron. Recently, in order to make the valve gear lightened, an Al alloy valve lifter has been used.
However, such an Al alloy valve lifter has lower mechanical strength and wear resistance than an iron lifter, so that an Al alloy cylinder which has the valve lifter allows sliding faces to wear earlier or brings about scoring. To prevent such disadvantages, the outer circumferential surface of the Al alloy valve lifter is coated with wear resistant material different from the base material. In this respect, it is necessary to increase adhesiveness of wear resistant material, thereby increasing adhesion strength to the base material. There is a method of coating the surface to form a rough surface and treating the surface with wear resistant material. To form the rough surface, before surface treatment such as thermal spraying is made, blasting is conventionally made with grit which has relatively large particle size.
Recently, allowable rotation speed and maximum brake power of an engine have been inclined to increase, and stress which acts on the circumferential surface of a valve lifter, such as bending or shearing stress, becomes excessive. A valve lifter in which the above Al alloy base material is coated with wear resistant material on the outer circumferential surface requires high adhesion strength at the coated layer.
However, in the above surface treating method by spraying after a single blasting, the coated layer has insufficient adhesion strength.
It is an object of the present invention to solve the above disadvantages and to provide a method of treating the surface of a valve lifter in which adhesiveness of a coated layer to base material is improved to increase adhesion strength, thereby providing a durable reliable valve lifter.